But Republicans remain skeptical about the hardship and urged Federal Aviation Administration chief Michael Huerta to find ways around furloughing controllers -- whose absence from the job would cause the delays.

Huerta said that flights through busy hubs such as New York, Chicago and San Francisco could suffer 90-minute delays because of furloughs anticipated for 10% of controllers. Those delays are expected to ripple across the rest of the country.

The full effect of the cuts is expected in April because controllers must be given 30 days notice about staffing changes.

Huerta told an American Bar Association conference that the 100 air-traffic control towers that will close and 60 that will lose midnight shifts will be chosen based on affecting the fewest people.

The towers will come from among those with less than 150,000 landings and departures each year, or 10,000 commercial landings and departures.

"We want to minimize the impact on the largest number of travelers," Huerta said. "It's for that reason that we need to start at the lowest-level facilities, meaning the smallest facilities that have the fewest operations."

The cuts appear likely because congressional Republicans, who have resisted changing the $85 billion in cuts across the government, argue that the administration could find the 5% savings in ways that don't compromise important functions such as airline flights.

At a House Transportation subcommittee hearing Wednesday, Republicans argued that FAA could reduce spending such as $500 million for contracts and $200 million for travel and supplies without resorting to furloughs.

"The history of the FAA has been one of financially stumbling and bumbling," Rep. Bill Shuster, R-Pa., chairman of the Transportation Committee.

Rep. Sam Graves, R-Mo., added: "The sky isn't falling."

Huerta said the largest contract for $228 million is for telecommunications necessary for controllers and that travel funding is important for safety inspectors. He said that after a hiring freeze and some reductions in travel, furloughs will still be necessary.

"Right now, based on where I am, based on where our contracts are, I don't see a way to avoid it, but we will work on it," Huerta said.

Nicholas Calio, president of the trade group Airlines for America, told the American Bar Association conference that his group is urging Congress and the administration to adopt more targeted spending cuts that avoid hurting airlines.

"In the last week and a half, we are the golden child," Calio said. "If there is no deal, we are urging the FAA to implement cuts that will have the very least impact on our customers and shippers."

The National Air Traffic Controllers Association, the union representing the controllers, issued a 14-page report detailing the hubs where the delays could be worst.

Chicago and Houston could lose more than one-third of their flights by using two runways rather than three, the report said.

"The ripple effect would be significant," Paul Rinaldi, the union president, told an Aero Club luncheon Wednesday. "If they proceed, these indiscriminate cuts will ultimately result in fewer flights and increased delays, creating a ripple effect that will hurt airlines, pilots, flight attendants, private aviators, airport employees, passengers and the many businesses, large and small, that depend on a vibrant aviation sector to survive and thrive."

Huerta explained to lawmakers that at Chicago's O'Hare airport where there are two towers, reducing staffing could mean closing one tower -- and basically closing a runway in bad weather.

"It runs at a very tight level of staffing," Huerta said. "If we close the north tower, that effectively removes a runway from operation."

Chicago's Midway airport is one that could lose its midnight shift of air-traffic controllers. At closed towers, planes could still land and take off on their own, but Huerta said flights can be slowed down in bad weather and by ensuring that a plane has actually landed before another one takes off.

"If you don't have a tower at a facility, it's going to be less efficient," Huerta said.